Great idea. Since a bunch of us don't do a lot of them, "any thematic map" might be a good enough focus. But even the Wikipedia explanation leaves a lot of room for judgment. I mean, were the April 2010 contest entries thematic? Worldwide distribution of beers.... When I think of thematic maps there's something more than just spatial distribution - else a regular political map would be "thematic, showing distribution of population"....

The dasymetric map shown as one of five types of thematic maps in that Wikipedia article looks a lot like a climate zone map, but it shows values along a range - plant hardiness being a sort of linear progression. So would a typical climate map count?